Method and device for gravimetric dosing bulk material

ABSTRACT

In order to increase the dosing accuracy of a dosing device, the bulk material is temporarily stored upstream from a reservoir ( 1 ) in an intermediate storage container ( 6 ) and is transferred only when a compensating pressure value is reached. The invention also relates to a corresponding device with a valve lock ( 4,5 ) on the intermediate container ( 6 ).

DESCRIPTION

The invention relates to a method and an apparatus for gravimetricdosing bulk material with the preamble features of claims 1 and 8,respectively.

Such a method and an apparatus for charging a container with a powderymaterial is known from DE 34 13 757 A1. In this case, at least onefeeding pipe opens into a reservoir, through which the powdery bulkmaterial is successively drawn from one or several storage containers,is weighed in an additive manner there and, after the opening of a floorlock, is supplied to a mixing device. Such mixing units for dosingbatches of powdery goods are used in the ceramics industry for example,with several assortments of different bonding agents, fillers, dyes andadditives being weighed. The bulk materials are supplied to the weighingcontainer via conveying worms or a pneumatic conveyor stream. Theweighing precision substantially depends on the pressure conditions insuch weighing apparatuses which are situated in a pneumatic conveyorstream. For a sufficient weighing precision, pressure surges occurringin the conveyor stream, through the actuation of valves for example,that affect the weighing machine must be prevented. Frequently, theentering and exiting conduits for charging and discharging are guided ina horizontal manner in the connection region so that changes in pressurewill not exert any influence on the weighing cell. These measures areinsufficient to exclude any influence of the pressure conditions on theweighing signal, so that the weighing results can be altered even in thecase of low pressure differences or undefined leakage air quantities.

An adjusting method for a weighing container is known from DE 33 10 735A1, with the effect of power shunts on the weighing result beingminimized in such a way that from the apparent change in load or weightof the weighing container a corrective variable is determined based onthe temperature or pressure which is then superimposed on the weighingsignal by addition or subtraction. Since a direct influence is made onthe weighing signal, the ability to calibrate such a system is notgiven.

From EP-B-0 644 406 of the applicant, the arrangement of a pressuresensor for measuring the pressure in the weighing container is knownwhich allows detecting the respectively prevailing pressure conditionsin the weighing container. The weighing signal of the weighing cellswill be accepted by the electronic weighing system for registration onlyon reaching or falling below a certain threshold pressure value. Thisprevents that a weighing will occur under an excessive negative orexcess pressure which could alter the weighing results of the calibratedweighing cell and electronic weighing system. The registration of theweighing results and the forward switching to the next dosing step doesnot occur until the pressure in the weighing container substantiallycorresponds to the ambient pressure. This system is hardly suitable forthe pneumatic discharge, especially in the case of low conveying streamswith respectively low air quantities.

The invention is therefore based on the object of providing a method forthe gravimetric dosing of bulk material and a respective apparatus, witha higher weighing precision being achieved, especially at smallconveying streams.

This object is achieved by a method with the features of claim 1 and byan apparatus with the features of claim 8.

In accordance with an embodiment according to the invention, anintermediate container with a valve lock upstream of the weighingcontainer or reservoir for the gravimetric dosing apparatus is used tocontrol in a purposeful manner the bulk material supply and the chargingof the reservoir. This ensures that no leakage air can escape from thepneumatic conveyor system, especially during the start of the dosingprocess. This compensation of the actions of force by the conduitscharging and discharging the gravimetric dosing apparatus lead to anoverall more precise dosing.

Further advantageous embodiments are the subject matter of thesubclaims.

An embodiment is described and explained in closer detail by referenceto the enclosed drawing. The (only) FIG. 1 shows an embodiment of acontainer weighing machine, with a reservoir 1 of a gravimetric dosingapparatus 20 being supported through at least one weighing cell 2 whichis arranged in this case below a frame. The output of the weighing cell2 is connected with an electronic weighing system 3 with an integrateddosing control (cf. dot-dash arrow) which registers, processes anddisplays in a control computer the weight of the reservoir 1 and theweight running through the dosing apparatus 20. The dosing control unitalso activates a lower flap 4 in a feed pipe 1 a which converges into atubular intermediate container 6 which is connected with a storagecontainer (silo) situated above the same. A slide not designated incloser detail and an upper flap 5 are arranged below the storagecontainer 7 which can both be actuated by the electronic weighing system3 or the dosing control unit. The intermediate container 6 with theflaps that can be actuated in an alternating manner thus form aso-called valve lock.

A discharge flap 10 or stop slide valve is provided below the storagecontainer 1, which flap or slide valve is also activated by the dosingcontrol unit and is connected to a gravimetric dosing apparatus 20 withpneumatic discharge, especially a dosing rotor weighing machine. Thereservoir 1 of the dosing rotor weighing machine 20 is thus filled by avalve lock which is formed by the two flaps 4 and 5 and the intermediatecontainer 6 and works as follows:

At first the upper flap 5 of the intermediate container 6 opens. Bulkmaterial such as coke dust flows from the silo 7 (when the upper slideis opened) to the intermediate container 6 which is situated between theupper flap 5 and the lower flap 4. In exchange thereto, the displacedair is guided into the silo 7. The upper filling flap 5 closes over timeor by the signal of a filling probe of the intermediate container 6. Thepressure in the coke-filled intermediate container 6 is detected by apressure sensor 9 and the pressure signal is guided to the electronicweighing system 3, so that the pressure is brought by means of apressure control device integrated in the electronic weighing system 3to a pressure of the reservoir 1 at or slightly over the pressure asmeasured by the pressure sensor 29 by supplying compressed air via apump 16, which pressure is obtained especially by leakage air from apneumatic conveyor line 11 when the dosing apparatus 20 is emptied. Thelower flap 4 then opens following a command given by the electronicweighing system 3. The coke dust is thus discharged through the fillingpipe 1 a into the reservoir 1 situated beneath the same. The lower flap4 on the intermediate container 6 closes again after a short period oftime or after the empty report of the filling level probe 8. The airpressure in the intermediate container 6 as transmitted by the reservoir1 is then relieved optionally by opening a relief valve 12. This cycleis repeated in time-varying intervals, according to the follow-up flowbehavior of the material and the demanded dosing output of the dosingrotor weighing machine 20 and the filling level requirements asdetermined from the respective weight of the reservoir 1.

The coke dust then flows continuously from the reservoir 1 to the cellwheel of the dosing rotor weighing machine 20. As already indicatedabove, the cell wheel or dosing rotor can swivel about axis 18 byresting on or suspended from the weighing cell 2 and thus supplies aweighing signal. Like the storage container 7, the reservoir 1optionally comprises a ventilation device 13 for fluidizing the cokedust. For the additional support of the material flow, a motor-driven,continually operating stirrer arm 1 b is installed in the reservoir 1.The filling level in the reservoir 1 is thus gravimetrically detected bythe measuring device and its filing is regulated accordingly bytriggering the valve lock 4, 5.

As stated above, it is relevant to provide a pressure sensor 9 on theintermediate container 6 in order to detect the pressure prevailing inthe same and to balance the same with the pressure in the reservoir 1 asdetected via the pressure sensor 29. The pressure sensor 9 like thepressure sensor 29 is connected to the electronic weighing system 3, inwhich a specific threshold pressure value is determined and, dependingon the demanded precision, is saved as a set-point value, preferably apressure difference of close to zero. Only when the pressure sensor 9 ofthe intermediate container 6 detects a pressure in comparison with thepressure value of the pressure sensor 29 on the reservoir 1 whichcorresponds to this compensation or limit differential pressure valuepreferably close to zero, will the electronic weighing system 3 emit thesignal to open the lower flap 4 and thus the dosing of a further batchis initiated into the reservoir 1 which is thus close to the samepressure level.

Moreover, a further pressure sensor 19 can detect the pressureconditions in the pneumatic discharge apparatus 11 in order to mutuallyadjust the pressure signals with those of pressure sensors 9 and 19 inthe electronic weighing system 3. Since a relatively high pressure canbe achieved in the intermediate container 6 in charging with powderybulk materials even in the case of low volume flows, it is appropriateto provide there at least one venting apparatus 12 for the purpose ofquicker pressure reduction, so that the desired limit pressure value andin particular the complete pressure balancing is achieved as rapidly aspossible. A filter is used for example as a venting apparatus 12 inorder to prevent any escape of powdery material. A similar pressurerelief apparatus 17 is also provided on the reservoir 1 in order toachieve a pressure reduction there too for adjustment to the pressure inthe intermediate container. Conversely, there can also be apressurization of the containers 1 and 6 through the compressed air pump16 in order to achieve a pressure balancing or adjustment prior to theopening of the lower flap 4.

In this embodiment with pneumatic conveyor apparatus 11, leakage air andwrong flows with falsifications of the weighing results can no longeroccur as a result of the compensation of the pressure conditions in thereservoir 1 and the intermediate container 6 with valve lock 4, 5 viathe discharge apparatus. As a result of the pressure measurement withthe pressure sensors 9 and 29 or 19, the conveyance of the bulk materialof the dosing rotor weighing machine 20 can be optimized. If there is anexcess pressure in the reservoir 1, the intermediate container 6 can bepressurized through the compressed air pump, thus leading to a pressureadjustment relative to the reservoir 1. This adjustment of the pressureconditions in the intermediate container 6 and in the reservoir 1 allowsincreasing the dosing precision by avoiding wrong flows and return flowsof particles in the dosing apparatus 20.

The opposite situation occurs analogously in a negative pressure in thereservoir 1. A negative pressure in the reservoir 1 can occur forexample when the discharge apparatus 11 is configured as a suctionapparatus. A pressure compensation by venting or pressurizing thereservoir 1 or the intermediate container 6 can be achieved before theopening of the lower flap 4 occurs for forwarding the bulk material.

1. A method for gravimetric dosing of bulk material which is suppliedfrom a storage container to a reservoir, is weighed there by determininga weighing signal for forwarding to an electronic weighing system and isdischarged from the reservoir to a gravimetric dosing apparatus, whereinthe bulk material is intermediately stored in an intermediate containerprior to the weighing in the reservoir and is only released uponreaching a compensation pressure value.
 2. A method as claimed in claim1, wherein the pressure in the intermediate container and in thereservoir is measured continually.
 3. A method as claimed in claim 1,wherein the intermediate container and/or the reservoir is vented from acertain pressure increase rate.
 4. A method as claimed in claim 1,wherein in the case of a pneumatic charging/discharging of the bulkmaterial with negative or excess pressure, the pressure in the conveyingconduit is measured in addition and is compared with the pressure in theintermediate container for determining the compensation pressure value.5. A method as claimed in claim 4, characterized in that thecompensation pressure value is fixed to a pressure difference ofapproximately zero.
 6. A method as claimed in claim 1, wherein thepressure measured in the intermediate container and/or the reservoir,and in particular their differential pressure value, is guided to adosing control unit or electronic weighing system for the purpose ofcontrolling a dosing apparatuse.
 7. A method as claimed in claim 6,wherein the intermediate container and/or reservoir can be pressurizedby a pump.
 8. An apparatus for gravimetric dosing bulk material with areservoir which rests on at least one weighing cell connected to anelectronic weighing system and is connected via flexible connectionswith a bulk material supply line and a gravimetric dosing apparatus,characterized in that wherein a valve lock with an intermediatecontainer is arranged upstream of the reservoir.
 9. An apparatus asclaimed in claim 8, wherein at least one filling level probe is providedon the intermediate container.
 10. An apparatus as claimed in claim 8,wherein a pressure sensor is provided each in the intermediate containerand in the reservoir for detecting the pressure.
 11. An apparatus asclaimed in claim 9, wherein at least one filling level probe is providedon the intermediate container.